Automatic process control system



C. G. ROPER ET AL AUTOMATIC PROCESS CONTROL SYSTEM Aug. 16, 1960 v FiledNov. 2, 1953 ll Sheets-Sheet 1 ,L I 5 M MW www ma Q w W c Vv Nzmo am an.1, \Q .N d ,l a 3m ,mm @MKM /1 l\l l /f u- Q /Ar V.. NQ.\.\h\/| d@ MW BM @oo ohr/ @IEN QW L UNNENQMNW W.W\ im: mb lll ||||||Il .www @w mn. Nwww@ y @NNW Ehmwm. mw S3290 V wwmusn w\m\ \\wm. v sw.. 3, Xmw. S@ f .o jmmv m* Y En w Gv uw N Se N Nv N5 H uw M w @mm A S J m www@ wvvwb QQNNMWf) n n im Filed Nov. 2, 1953 Aug. 16, 19

C. G. ROPER ETAL AUTOMATIC PROCESS CONTROL SYSTEM l1 Sheets-Sheet 2flul'c flow @kades 6.

IN V EN TORS Aug. 16, 1960 c. G. RoPr-:R i-:TAL 2,949,273

AUTOMATIC PROCESS CONTROL SYSTEM Filed Nov. 2, 1953 ll Sheets-Sheet 3fasi/Herzi- 201 INVENToRs: `209 Charles koper 5 Aug. 16, 1960 c. G.RoPr-:R ETAL AUTOMATIC PROCESS CONTROL SYSTEM Filed Nov. 2, 1953 l1Sheets-Sheet 4 Aug. 16, 1960 c. G. RoPER ETAL AUTOMATIC PROCESS CONTROLSYSTEM 11 Sheets-Sheet 5 Filed Nov. 2. 1953 i@ 12- i?? if@ QON@ w Mew. 51 y A me@ m T wwwmain@ 7 TQQ s TT w um i H w+ ad t @E Y .EA/Nm, B mm..

Aug- 16, 1950 c. G. RQPER ErAL 2,949,273

AUTOMATIC PROCESS CONTROL SYSTEM l1 Sheets-Sheet 6 Filed Nov. 2, 1953INVENTORS Char/e5 6. koper: E, faynrz'lckrzsi BY 5 if 24o 'orn 5 Aug.16, 1960 c. G. RoPER ETAL 2,949,273

AUTOMATIC PROCESS CONTROL SYSTEM Filed Nov. 2, 1853 11 Sheeis-Slfxeet 7Aug. 16, 1960 c. G. ROPER ET AL AUTOMATIC PROCESS CONTROL SYSTEM llSheets-Sheet 8 Filed Nov. 2, 1955 Aug. 16, 1960 c. G. RoPr-:R ETAL2,949,273

AUTOMATIC PROCESS CONTROL SYSTEM Filed Nov. 2, 1953 11 Sheets-Sheet 9INVENTORS: Char/e5 Roper- Aug. 16, 1960 c. G. ROPER ETAL AUTOMATICPROCESS CONTROL SYSTEM 1l Sheets-Sheet 10 Filed Nov. 2. 1955 C) li IN VEN T0 G. Q05 Edf ar', Gl@ risZ' Char/cs Aug. 16, 1960 c. G. Rom-:R ETAT.

AUTOMATIC PROCESS CONTROL SYSTEM ll Sheets-Sheet 1l ,L eso Filed NOV. 2,1953 United States Patent Q AUTOMATIC PROCESS CCNTROL SYSTEM Charles G.Roper, Fairfield, and Edgar S. Gilchrist, Easton, Conn., assignors, bymesne assignments, to Robertshaw-Fulton Controls Company, Richmond, Va.,a corporation of Delaware Filed Nov. 2, 1953, Ser. No. 389,564

20 Claims. (Cl. 251-26) The present invention relates to an automaticcontrol system, more particularly, to a system for automaticallycontrolling one or more variable quantities of an industrial process,=and the invention has for an object the provision of a system of thistype wherein transmission lag is substantially eliminated and a highdegree of stability is achieved so as to permit the use of narrowproportional bands and to provide a system which is very sensitive tochanges in the measured variables.

It is another object of the present invention to provide a new andimproved automatic process control system wherein provision is made forchanging from automatic to manual control and vice versa withoutproducing an undesired discontinuity or bump in the process so as topermit service and repair of the automatic control units and manualadjustment of the process without interfering with the continuitythereof.

It is a further object of the present invention to provide a new andimproved automatic process control system having proportional-resetaction wherein the valve travel on changing from manual to automaticoperation is independent of the setting of the proportional band controlof the system and the changeover from manual to automatic control doesnot require increasingly critical adjustments as narrower proportionalbands are used.

It is a still further object of the present invention to provide a newand improved automatic process control system of the electronic typewhich may be completely energized from unregulated alternating currentsources While providing a high degree of stability and sensitivity.

It is another object of the present invention to provide a new `andimproved automatic process control system whereby a given controlarrangement may be expanded by providing an overall feedback loopwithout producing additive time lags through the system so that theaccuracy of control of the overall system is increased.

It is still another object of the present invention to provide a new andimproved automatic process control system of the electrical type whereina major control loop is employed and a series of minor control loops areindividually associated with dierent series components of the majorcontrol loopto improve the quality of these series components, saidmajor and minor loops each including degenerative feedback :arrangementswhereby the stability of gain and dynamic response of the system isimproved.

Another object of the present invention resides in the provision of anew and improved automatic process control systern wherein controlcomponents are standard and are interchangeable at the control panelregardless of the type of variable under control.

Still another object of the invention resides in the provision of anautomatic process control system of the electronic type whereinelectro-pneumatic transducers are provided to tie into pneumatic controlsystems when required.

It is :another object of the present invention to prorv'ice vide 1a newand improved automatic process control system of the electrical type inwhich a series of control loops responsive to `different variables maybe cascaded and an indication of the set point of each variable isprovided.

It is still another object of the present invention to provide a new andimproved automatic process control system of the electrical type whichis particularly adapted to function with conventional analog to digitalcomputers and conventional calculating machines to provide monitoringand computation facilities for the process without substantialmodification of the control system.

It is still further an object of the present invention to provide a newand improved automatic process control system in which direct currentsignals are transmitted in both directions between the process andcontrol areas which are substantially free from fluctuations or noiseresulting from a carrier signal or signal chopping device which are inthe frequency range of useful control signals of the system.

It is another object of the present invention to provide ya new andimproved automatic process control system of the electrical type whereinthe same type of signal is provided for both input and output so thatseveral control units may be connected in series for cascade control.

It is a further object of the present invention to provide 'a new andimproved automatic process control system of the electrical type whereinthe input and output conductors of the system may be connected todifferent potentials while permitting the transmission of a directcurrent signal through the system.

`It is a still further object of the present invention to provide a newand improved automatic process control system wherein the output currentof the measuring and control components is stabilized against theetfects of load circuit resistance changes and the presence of voltagesources in the load circuit.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the ol lowing specification taken inconnection with the accompanying drawings, in which:

Fig. 1 is a block diagram of an automatic process control systemembodying the principles of the present invention;

Fig. 2 is a block diagram of an alternative process control systemembodying the principles of the invention wherein more precise controlis obtained by using a second set of control components to reduce thevariations of the controlled variable of the process.

Fig. 3 is an electrical circuit diagram of the transmitter unit in thesystem shown in Fig. l;

Fig. 4 is a side elevational View of a transmitter unit positioned in anexplosion proof housing and showing the mechanical features thereof;

Fig. 5 is a plan view, partly in section of the transmitter unit of Fig.4;

Fig. 6 is a fragmentary side elevational view of a portion of thetransmitter unit of Fig. 4, on a somewhat larger scale;

Fig. 7 is a plan View, partly in section, of the electromechanicalbalance used in the transmitter unit of Fig. 4;

Fig. 8 is a fragmentary side elevational View, in section, taken alongthe line S-3 of Fig. 7;

Fig. 9 is a fragmentary sectional view taken along the line 9 9 of Fig.7;

Fig. l0 is a sectional side elevational view taken along the line 10-10of Fig. 9;

Fig. ll is a fragmentary side elevational View taken along the line11-11 of Fig. 9;

Fig. l2 is a front view of the recording and set point unit of thesystem of Fig. l;

i Fig. 13 is a plan view of the chassis of the recordlng and set pointunit of Fig. 12; 13Fig. 14 is a side elevational view of the chassis ofFig.

Fig. 15 is a fragmentary plan view, in section, of the chassis of Fig.14 taken along the line 15--15 thereof;

Fig. 16 is a sectional side elevational view taken along the line 16-16of Fig. 14;

Fig. 17 is a fragmentary side elevational view, taken along the line17-17 of Fig. 16;

Fig. 18 is a fragmentary side elevational view, taken along the line 118of Fig. 16;

Fig. 19 is a sectional side elevational View taken along the line 19-19of Fig. 14;

Fig. 2l() is a schematic diagram of the electrical cir- 1cxuits 1of therecorder and set point unit of the system of Fig. 21 is a sideelevational view taken along the line 21-21 of Fig. 19;

Fig. 22 is a sectional plan View taken along the line 22-22 of Fig. 14;

Fig. 23 is an electrical circuit diagram of the electronic controllerand manual positioning units in the system shown in Fig. l;

Fig. 24 is a side elevational view of the valve positioning unit in thesystem of Fig. l shown in conjunction with a typical valve;

Fig. 25 is a somewhat diagrammatic view of the valve positioning unit ofFig. 24 showing the operation thereof;

Fig. 26 is a side elevational view of the valve positioning unit of Fig.24 with the cover thereof removed and a portion thereof broken away toshow the details of the air relay incorporated therein;

Fig. 27 is a bottom View of the valve positioning unit of Fig. 24 with aportion of the casing thereof broken away;

Fig. 28 is a side elevational view of a portion of the valve positioningunit of Fig. 26 on a somewhat larger scale;

Fig. 29 is a fragmentary side elevational view taken along the line29-29 of Fig. 28;

Fig. 30 is a fragmentary side elevational view on a somewhat largerscale, of the electromechanical balance unit and associated componentsused in the valve positioning unit of Fig. 26 with the cover of thebalance unit removed; and

Fig, 31 is a fragmentary sectional View taken along the line 311-31 ofFig. 30.

Referring now to the drawings and more particularly to Fig. 1 thereof,the automatic process control system of the present invention is thereinillustrated as controlling a typical industrial process indicatedgenerally at 39. In the illustrated embodiment, the process 39 maycomprise a stripping column which is used in a petroleum rening processwherein crude oil is introduced into the top of the column andsuperheated steam is introduced at the bottom thereof. The admittedsteam reduces the partial pressure of the lighter fractions in the crudeoil and these lighter fractions are evolved in a vapor which is carriedolf with the steam, the process being in part controlled by the valve 46in the oil outlet line. In this connection it will be understood thatthe process 39 is shown merely for purposes of illustration and theprocess control system of the present invention may be employed tocontrol any industrial process involving the measurement of processvariables and the control of the process in accordance therewith.

ln the system of the present invention `direct current signals, whichare substantially free from fluctuations which are in the frequencyrange of useful control signals of the system, are transmitted betweenthe process and control areas of the plant so thatthe transmission lagsnormally encountered in pneumatic control systems are substantiallyeliminated. In the illustrated embodiment, the valve 46 is controlled inaccordance with changes in the liquid level Within the column 39 asdetermined by a displacement type liquid level measuring deviceindicated generally at 40. More specifically, the illustrated embodimentof the process control system of the present invention comprises atransmitter unit 41 which is controlled by the liquid level measuringdevice 40, a recording and set point unit 42, an electronic controller43, a manual positioning unit 44 and an electropneumatic valvepositioning unit 45.

The transmitter unit 41, which is located in the process area, isenergized by an. unregulated alternating current supply and transmits adirect current signal over the conductors 47 and 48y to the recordingand set point unit 42 located in the control area. The process andcontrol areas may be separated by as much as thirty miles withoutintroducing any appreciable transmission time lag. Preferably, thedirect current signal which the unit 41 transmits to the unit 42 has astandardized range of from 0.5 to 5.0 milliamperes, and varies withinthis range in accordance with variations the liquid level within thestripping column 39. The transmitter unit 41 is provided with its owninternal feedback loop so that the stability and dynamic response of theunit 41 to changes in liquid level are improved and the unit 41 is.rendered less responsive to extraneous disturbances. Also, by providingan independent feedback loop within the transmitter unit 41, the outputcurrent thereof remains sub'- stantially constant despite Widevariations in line length and resistance of the output circuit so thatthe unit 41 does not have to be calibrated for each installation and maybe connected in parallel with the output of another transmitter unit orthe input of a control unit Without changing the output currents.

The recording and set point unit 42 is also energized by an unregulatedalternating current supply and includes a recorder portion whichresponds to the direct current signal transmitted thereto from thetransmitter unit 41 by recording the value of the measured variable onthe chart 50. The unit 42 is also provided with a pointer 51, attachedto the recording pen of the instrument, which cooperates with the scaleS2 to provide an indication of the magnitude of the controlled variableat any instant. In order to set the control point of the process, theunit 42 is also provided with a set point portion which includes 'a setpoint indicator 53 which may be manually adjusted to provide the desiredset point and the set point portion of tbe unit 42 lfunctions to developan error signal on the output conductors 55 and 56 thereof which isproportional to the deviation of the indicator 51, and hence thecontrolled variable, from the set point as determined by the position ofthe indicator 53. The recorder and set point portions of the unit 42 arecompletely independent from one another so that the set point of thesystem is not disturbed if the recording pen is held or movedaccidentally. The recorder portion of the unit 42 is provided with itsown separate feedback loop to provide accurate positioning of therecording pen and permit the recorder to respond to relatively highfrequency changes in the transmitter output signal.

In order to provide an error signal which is suitable for operationVwith vacuum tube circuits in the electronic controller 43, the setpoint portion of the unit 42 converts the variable direct current signaldeveloped by the transmitter unit 41 into -a variable voltage errorsignal which is proportional to the deviation of the process from theset point. Thus, if the recording indicator 51 is at the top of thescale 52 and the set point indicator 53- is at the bottom of the scale52 a positive error signal of approximately 22.5 volts D.C. is producedbetween the conductors 5,5 and S6. On the other hand, if the indicator51 is at the bottom of the scale 52 and the set point indicator 53 is atthe top of the scale, a negative voltage of approximately 22.5 voltsD.C. is produced between the conductors 55 and 56. The set point portionof the unit 42 is also provided with an independent feedback looptherein so that a constant voltage output circuit is provided for thetransmission of the error voltage between the units y42 and 43.Accordingly, the error voltage between the conductors 55 and 56 ismaintained substantially constant with large variations in the impedanceof the output circuit thereby permitting several controllers to beparalleled to the output conductors 55 and 56 or a low impedancemeasuring device to be connected thereacross.

The electronic controller 43 is energized from an unregulatedalternating current supply and comprises a balanced differential amplierwhich is provided with a proportional band control 61, a reset ratecontrol 62 and a rate response control 63 which may be adjusted tomodify the error voltage produced between the conductors 55 and 56 toprovide for any desired mode of control of the process. The controller43 also converts the error voltage developed on the conductors 55 and 56into a direct current valve positioning signal which is transmitted overthe conductors 59 and 60 to the manual positioning unit 44. Preferably,this valve positioning signal has the same standardized range of from0.5 to 5.0 milliamperes as the transmitter unit 41. The controller 43 isprovided with a feedback loop between the output and input of thebalanced differential amplifier so that the output current supplied tothe conductors 59 and 60 is held substantially constant over a widerange of output impedance. Accordingly the output of the controller 43may be connected in parallel with the output of a transmitter unit forcascade purposes and the output current of the controller 43 is renderedsubstantially independent of line length and resistance.

The manual positioning unit 44 is energized from an unregulatedalternating current supply and includes a stable direct current sourcewhich may be used for manual control. The unit 44 is provided with amanualautomatic changeover switch 65 to permit shifting the system fromautomatic to manual operation. When the switch 65 is in the automaticposition the valve positioning current developed on the conductors 5%and 60 by the unit `43 is transmitted through the unit 44 withoutmodification and over the output conductors 66 and 67 thereof to theelectro-pneumatic valve positioning unit 45 which is located in theprocess area. In the unit 45, the valve positioning current is convertedinto a corresponding pneumatic control signal which is derived from theair supply line 6E, and is employed to actuate the pneumaticallycontrolled inal control valve y46 in the proper direction to reduce theerror signal to zero. The unit 45 is provided With its own independentfeedback loop so that the valve 46 is positioned in precise relationshipto the valve positioning current transmitted over the conductors 66 and67 regardless of stem loading on the valve and variations of pressure inthe supply line 68.

In order to change over from automatic to mnaul operation the manualpositioning unit 44 is provided with a meter 7G, which indicates thevalue of -the valve positioning current transmitted over the conductors65 and 67 to the valve positioning unit 45, and the manual positioningunit 44 is also provided with a manual positioning control knob 7l. Theposition of the control knob 71 determines the current supplied 'fromthe direct current source in the unit 44 to the conductors 66 and 67when the unit 44 is in the manual position and the control knob 71cooperates with a scale 72 which is calibrated in terms of the manualcontrol current transmitted over the conductors 66 and 67 when the unit44 is in the manual position. Accordingly, when it is desired to changeover from automatic to manual operation the control '71 is adjusted sothat the position of the pointer thereof on thel scale 72 matches theneedle of the indicating meter 70, after which the changeover switch 65is thrown to the manual position and the current transmitted over theconductors 66 and 67 is thereafter controlled solely by the position ofthe manual positioning control 71 andv is unaffected by the automaticcontrol components 41, '42 and 43. When it is desired to change back toautomatic control the switch 65 is merely thrown back to the automaticposition and the process is thereafter controlled in accordance with theposition of the set point indicator 53 and the mode of control set up inthe electronic controller 43.

In order that the transfer from manual operation back to automaticoperation may be made without critical manual adjustments of thecontroller and manual positioner Whiie at the same time minimizingabrupt variations in valve travel if an error exists on changeover, acontrol voltage proportional to the setting of the manual positioningcontrol 711 is transmitted over the conductor 75 from the unit 44 to theelectronic controller 43. A relay in the controller 43 is energized overthe conductors 57 and 58 when 'the switch `65 is in the manual positionso that during manual operation the control voltage on the conductor 75is impressed upon the controller 43 and continuously readjusts thecontroller in accordance with the value of the manual control current.ln order to minimize valve travel on changeover when the process is notprecisely on the set point, the controller is automatically readjustedin a manner not dependent on the position of the proportional bandcontrol 61. Accordingly, when it is desired to change from manual toautomatic operation the switch 65 is merely thrown to the automaticposition and, since the controller 43 is continuously readiusted duringmanual operation, the maximin-n valve motion on changeover when an errorexists will be `one-half the deviation of the process from the set pointestablished by the unit 42, regardless of the setting of theproportional band control 61, and the phenomenon known as bumping theprocess is held to a minimum. Furthermore, valve motion on changeovermay be substantially completely eliminated if the switch 65 is thrown tothe automatic position at a time when the process -is `on set point.

lt will be evident from the above general description of the system ofFig. l that an arrangement is provided in which individual units may beenergized from unregulated power lines so that the process can beeconomically controlled at any desired control area and the operation ofan entire plant can be coordinated at one central location, Furthermore,the system of the present invention is particularly adapted to functionwith conventional analog to digital computers and conventionalcalculating and punched card machines without increasing the cost orcomplexity of the process control system so that monitoring andcomputation operations may be performed at convenient locations withoutinterfering with the process control system, per se. Thus, if it isdesired to provide read and print out facilities for the variablemeasured fby the transmitter 41, a small resistor is inserted in serieswith one of the output conductors 47, 43 so that the direct currenttransmitter output Hows through this resistor. Since conventional analogto digital computers and read out devices are designed to work from lowlevel D C. voltages, the voltage developed across the series resistor,which will be in the order of millivolts, may be employed directly asthe input signal for the monitoring device. Also, since the outputcurrent from the transmitter is stabilized, a large number of outputsignals may be derived for different ancillary functions withoutchanging the value of the transmitter outputhcurrent. In thealternative, a large number of transmitters, each measuring a diiierentvariable may be scanned successively by means of a suitable switchingarrangement for connecting the series resistors of successivetransmitters to the input of the monitoring or calculating device.

lt will also be noted that in the system of Fig. l the input and outputconductors to the control area may be connected to different potentialsWhile permitting the transmission of a direct current signal through thesystem. With this arrangement the input conductors 47, 4S may beconnected to a different potential than the output conductors 66, 67without interfering with the operation of the system. For example, inmany installations it is desirable to use leased telephone lines for thetransmission `of signals between the process and the control areas.Accordingly, either the input conductors or the output conductors, orboth, may be connected to telephone lines running between the processand control areas on which different potentials appear due to thetelephone system itself and the transmission of direct current signalsthrough the process control system is unaffected. It will also :be notedthat one conductor between the process and control areas may be used `asa common conductor for a large number of input or output direct currentsignals so that a substantial saving in installation and maintenance isachieved with the electrical transmission system ofthe presentinvention.

In order to illustrate the flexibility and versatility of the componentunits of the automatic process control system `of the present inventionthere is shown in Fig. 2 an alternative process control system whereincascade control of two variables `of the system is provided. In thesystem illustrated in Fig. 2, a typical process control system has beenshown -wherein the control agent of the process is steam which isemployed to apply heat to the process iluid lby means `of the heatexchanger 100. The amount of steam entering the heat exchanger 100v iscontrolled by the valve l101 and the temperature of the process luid ismeasured by means of the thermocouple 102 which is placed in the outletprocess liuid line of the heat exchanger 100, the thermocouple 102 beingineluded in the major control loop of the process. The process alsoincludes a minor control loop which includes the oriice 103 in the steaminlet line and the differential pressure cell 104 which are employed todetect changes in the steam ilow rate so that the process is controlledboth as to the amount of heat added to the control medium and the ilo-wrate of the control agent i.e., incoming steam. The major control loopof 4the process includes a temperature transmitter `105, a recording andset point unit 106, an electronic controller 107, a manual positioningunit S, a recording and set point unit 109, an electronic controller110, a manual positioning unit 111 and an electro-pneumatic valvepositioning unit 112, the unit 112 functioning to position thepneumatically controlled valve 101 in the steam inlet line. The units109, 1110, 111 and 1112 are common to both control loops of the processand the minor control loop also includes a differential pressuretransmitter 113 which is connected over the conductors 145 and 146 tothe recording and set point unit 109. The units 106 to 112, inelusive,may all be substantially identical to the corresponding units in thesystem of Fig. 1. However, in the system of Fig. 2 the component unitsyare interconnected in a somewhat different manner to provide forcascade control of the process.

The temperature transmitter 105 which is energized from an unregulatedalternating current supply, receives an electrical signal from thethermocouple 102 over the conductors 115 and 116 and converts thethermocouple input into the standard transmitter signal of the system,i.e., a direct current signal within the range of from 0.5 to 5.0milliamperes. This transmitter signal is transmitted over the conductors117 and 118 to the recording and set point unit 106 which is located inthe control area. In the unit 106 the value of the measured variablei.e., the temperature of the process fluid is indicated on the chart 120and the major set point of the system may be adjusted by moving the setpoint indicator `121 of the unit 106. The unit 106 develops an errorvoltage signal between the conductors 123 and 124 thereof which isproportional to the deviation of the process from the established setpoint and is transmitted over these conductors to the electroniccontroller 107. The controller 107 is provided with a proportional bandcontrol 125, a reset rate control 126 and a rate time control 127 sothat any desired mode of control of the temperature of the process iiuidmay be set up and the controller 107 transmits a valve positioningcurrent over the conductors 128 and 129 to the manual positioning unit108i. When the unit I108 is in the automatic position this valvepositioning current is transmitted over the conductors 130 and 131 tothe recording and set point unit 109. More particularly, the valvepositioning current developed by the electronic controller 107 isconnected by way of the conductors 130` and 131 to the set point inputterminals and 136 of the unit 109 so that this valve positioning currentacts as an electrical set point signal for the set point portion of theunit 109 but does not alect the recording portion of the unit i109.

The diierential pressure transmitter '113, which is energized from anunregulated alternating current supply, is controlled in accordance withthe position of the output shaft of the differential pressure cell 104and transmits a direct current signal, which varies within the standardrange of from 0.5 to 5.0 milliamperes, over the conductors and 146 tothe recording and set point unit 109* in the control area. Thetransmitter 113 is provided with an independent feedback looptherewithin so that the output current thereof 4is stabilized againstload circuit variations. The direct current signal from the transmitter113 is connected to the terminals 132 and 136 of the unit 109 so as tobe applied to both the recording and set point portions of the unit 109in series. As a result, variations in the rate of flow are recorded onthe chart 147 in the unit 109 and the set point portion of the unit 109responds to the direct current signals from the units 108 and 11-3lwhich are connected in opposition across the set point input terminals135 and 136.

The controller 110 is provided with a proportional band control 151, areset rate control 152 and a rate response control and functions in amanner similar to the controller 107 to convert the error signaldeveloped by the unit 109 into a corresponding valve positioning currentwhich is transmitted over the conductors 139 and 140 to the manualpositioning unit 111. When the changeover switch 155 in the unit 111 isin the automatic position this Valve positioning current is transmittedover the conductors 141 and V142 to the valve positioning unit 112 inthe process area. Accordingly, the position of the valve 101 iscontrolled in such manner as to provide a iiow rate corresponding to thevalue of the output current from the manual positioning unit 100.Preferably, the set point indicator 170 of the unit 109 is moved to aposition below the zero point of the scale 171 which corresponds to zerocurrent input. In the illustrated embodiment wherein a current range of0.5 to 5.0 rnilliamperes is used, the set point indicator 170 would bemoved to a point approximately 11% below the Zero point of the scale171. When the set point indicator 170 is in this position, theelectrical signal from the controller 107 corresponds to the rate offlow through the valve 101 since the unit 109 does not introduce anymechanical set point component. Accordingly, whe-n the indicator 170 isin the rdescribed position, the meter in the manual positioning unit10S, through which the output current from the controller 107 flows,provides a direct and continuous indication of the set point of theminor control loop. Thus, if the meter 160 reads half scale the operatoris informed that a 50% ow rate is being maintained. Since the set pointindicator 121 in the unit 106 also continuously indicates the set pointof the major control loop it will be evident that the system of Fig. 2provides continuous indications for all set points in a cascade controlsystem.

In the system of Fig. 2 the maior control loop normally functions sothat the rate of steam flow produces the desired temperature and thisflow is adjusted by means of the valve 101 in accordance with thesetting of the set point indicator 121. However, changes in the steampressure head due to outside demands and other similar disturbances inthe steam inlet line may introduce errors in the control of the processsince the temperature control arrangement of the major control loop willnot respond quickly enough to these short time fluctuations.Accordingly, the minor control loop which includes the transmitter 113,the recording and set point unit 109, the electronic controller 110 `andthe manual positioner 111, is provided to improve the time response andaccuracy of flow adjustment so that the effects of changes in steampressure head are substantially reduced. In effect, the minor controlloop comprises a means for improving the performance of the flow rateadjustment which is itself a series component in the major control loopof the system. Since the transmitter 113 serves as the feedback elementin the minor control loop, the response to changes in steam pressurehead will be limited only by the dynamic response of the unit 113 andthe adjustment of ilow rate by the major control loop will be performedin accordance with temperature variations of the process andsubstantially independently of variations of flow rate due to outsidedisturbances.

yIf it is desired to change over the major loop to manual control, themanual positioning control 161 in the unit 108 is matched to the needleposition of the meter 160 and the changeover switch 162 is actuated tothe manual position. When this occurs, the minor control loop assumescontrol insofar as automatic operation is concerned so that the units S,10o and 107 in the major control loop may be removed for service orrepair. During this period the set point portion of the unit 109automatically controls the process solely in accordance with changes inthe steam How rate. lf both major and minor loops are to be removed, themanual positioning control 154 in the unit 111 is matched to the needleposition of the meter 153 and the changeover switch 155 is thrown to themanual position so that all of the units except the units 111 and 112are rendered ineffective to control the process. However, when thesystem is changed back to automatic operation each of the controllers1137 and 11d is controlled by the associated manual positioning unit sothat abrupt variations in valve movement are held to a minimum in theevent that set point errors exist, as described in detail heretofore inconnection with the system of Fig. l.

It will be noted that in the system of Fig. 2 the output of thetransmitter 113 is connected in parallel with the output of thecontroller 1117 across the set point input terminals 135, 136 of theunit 109. However, both of the units 113 and 167 are provided withinternal current stabilizing feedback loops so that the output currentof each unit is not aected by the loading of the other unit and cascadeconnection of control units may be made. Furthermore, since the inputand output signals of the system are both direct current signals of thesame standardized range of current values, several control loops may becascaded directly without requiring separate converting units betweencontrol loops of the system. It will also be noted that in the system ofFig. 2 the output current from the controller 107 may be applieddirectly to the input of the set point portion of the unit 109 and actsas an electrical reference level therefor. ln this connection, it willbe understood that the cascade system shown in Fig. 2 may be expanded toinclude control of one or more additional variables, which may be inentirely different areas than the transmitters 1t5 and 113, and byproviding overall feedback loops in each control loop additive time lagsare avoided so that the accuracy of control is greatly increased.

Transmitter unit 41 Considering now, in more detail, the transmitterunit 41 of the system shown in Fig. 1 reference may be had to Figs. 4 to11, inclusive, of the drawings, wherein the details of the unit 41 areshown, and to Fig. 3 of the drawings wherein the electrical circuitdiagram of the unit 41 is shown. Referring more particularly to theselfigures, the unit 41 comprises an electromechanical -balance unitindicated generally at 200 which is mounted on the sub-chassis 201within an explosion-proof case 2112 provided with a threaded cover 203.The output shaft 205 of the liquid level measuring device 40 isrotatably mounted in one wall of the housing 202 by means of the bushing206 and as the liquid level within the stripping column changes theinner end 207 of the shaft 205 is rotated in proportion thereto. Theshaft 205 is connected by means of a Zero and span adjustment linkageindicated generally at 208 (Fig. 6i) to the input shaft 209 of theelectromechanical balance unit 200 so that the shaft 209 is rotated inproportion to rotation of the input shaft 205.

Considered generally, the electromechanical balance unit 200 comprises aresiliently mounted metallic beam 210 (Fig. l0) which is deflected byrotation of the input shaft 209 thereof. More particularly, the shaft209 is rotatably mounted in the bushing 232 which is mounted on the base251 of the balance unit 200 and the shaft 209 is biased by means of thespiral return spring 233 the other end of which is secured to the post234 on the base 251 and the inner end of which is secured to the shaft209. A stop pin 241 is secured to the inner end of the shaft 209 and isadapted to engage the post 234 and prevent overtravel of this shaft. Theinner end of a spiral calibration spring 242 is secured to the shaft 209and the outer end of the spring 242 is connected through the twisted arm243 to the beam 210. Accordingly, as the shaft 209 is turned, the spring242 produces a corresponding torque on the beam 210 tending to deflectthe same. Deflection of the beam 210 causes the end portion 211 thereofto move toward a spiral wound planar inductor 212 which is connected tothe input circuit of a high frequency oscillator so as to vary thetuning thereof. The end 211 of the beam 210 carries on the undersidethereof a disk of insulating material 213 to which is secured aself-supporting single layer solenoid-wound coil 214 which is positionedwithin an annular air gap 215 formed in a permanent magnet structurewhich includes a central pole piece 216, the end plates 217 and 218, anda toroidal member 219 of magnetic material which are assembled in themanner shown in Fig. l0 to deline the annular air gap 215. A feedbackcurrent proportional to the oscillator output current is applied to thecoil 214 in the correct polarity to deflect the beam 210 in oppositionto the input torque which is applied to the beam through the input shaft209. Accordingly, the position of the lbeam 21) varies only by an amountsullicient to produce a change in the oscillator output current which isof sufcient magnitude to offset the torque input applied to the beam 210and rebalance the beam.

Considering now the manner in which the beam 210 is resilientlysupported on the upper end plate 217 of the magnet structure, a crossbar220 is spaced above the end plate 217 by means of the spacers 221 and222 and the bar 220 is provided with the notched portions 223 and 224 soas to permit the center portion of the bar 220 to be twisted out of theplane of the end portions of the bar 220. A transversely extendingmember 225 is clamped between the middle portion of the crossbar 220 andan upper plate 226 by means of the screws 227 and 228 and is providedwith a down turned edge portion 229 to the ends of which are secured theflat resilient supporting strips 230 and 231 which comprise the solesupporting elements for the beam 210. The bottom ends of the strips 230and 231 are secured to the ends of a transversely extending member 235which is secured to the central portion of the beam 210 as is bestillustrated in Fig. 10.

The position of the central portion of the bar 220 may be varied bymeans of the adjusting screw 236 which threads through the bar 220 andengages the upper surface of the end plate 217 so that the centerportion of the bar 220 may be twisted by the desired amount by means ofthe screw 236. Also, the position of the member 225 between the bar 220and the clamping plate 226 may be Varied as desired so as to provide aninitial adjustment of the beam 210 relative to the permanent magnetstructure so that the coil 214 may be centered within the air gap 215 inthe floating position of the beam 210. The beam 210 is counterbalancedby means of the counterweight 237 which is secured to the right hand endof the beam as viewed in Fig. l0, and a bracket 238 is provided adjacentthe weight 237 Which carries a horizontal balance adjusting screw 239'and a vertical balance adjusting screw 240. The positions of the screws239 and 240 relative to the horizontal and vertical axes of the beam 210may be varied by threading the same through the bracket 238 by thedesired amount so that the beam is dynamically Abalanced in both axes.With this arrangement the electromechanical balance unit 200 may be usedin any position without changing the lloating position of the beam orthe calibration thereof.

The planar inductor 2'12 is adjustably positioned above the end 211 ofthe beam 210 on a block 245 of insulating material which is supported onthe `screws 246 which extend upwardly from the end plate 217 of themagnetic structure, the U-shaped spring clips 247 being employed to biasthe block 245 upwardly against the adjustment nuts 243 on the ends ofthe screws 246. The ends of the coil 212 are connected to the terminals249 and 250 which extend through the base plate 251 of theelectromechanical balance unit 200. A cover 252 encloses the componentsof the vbalance unit `and is secured to the base member 251 by anysuitable means so that a completely enclosed, dust proof unit isprovided.

Considering now the details of the linkage 208 which is used to connectthe input shaft 207 to the shaft 209 of the balance unit 200, thislinkage comprises a motion transmitting arm 255 (Fig. 5) having anoffset portion 256 which is clamped to the end 207 of the input shaft205 and is provided with an offset portion 257 at the other end thereofwhich is connected to a span adjustment linkage including the upper link258 and the lower link 259 which are adjustably connected together bymeans of the lock screw 260 so that the length of the span adjustmentlink may be varied. On the other end of the input shaft l209 there isprovided a clamping plate 261 which is secured to the shaft 209 by meansof the lock screw 262 and a toothed segment 263 may be rotated withrespect to the plate 261 by means of the adjustment screw 264 and islocked in position by means of the lock screw 265. A slide 266 ispositioned on the segment 263 by means of the guide screws 267 and 268and the slide 266 is provided with an oset portion 269 which isconnected to the lower end of the span adjustment link 259.

From the foregoing description of the linkage 208 it will be seen thatas the input shaft 207 is moved in accordance with changes in themeasured Variable, which, in the illustrated embodiment, is the liquidlevel of the process, the offset end portion 257 of the arm `255 isrotated in an arc about the shaft 207 and this motion is transmittedthrough the `span adjustment linkage 258, 259 to the slide 266 so thatthe input shaft 209 of the balance unit 200 is rotated in proportionthereto. In this connection it will be seen that due to the offsetportion of the slide 266 the motion transmitting linkage may be rotatedthrough a substantial arc to accommodate a wide range of input shaftmovement. Adjustment of 12 the link 258, 259 determines the startingangle and hence linearity while the slide 266 may be adjusted withrespect to the segment 263 at the 4factory to provide a coarse spanadjustment of the instrument so that a standard input shaft movementproduces the correct arcuate movement of the input shaft 209. Inaddition the zero position of the linkage may be adjusted by means ofthe adjustment screw 264 so that in the zero position of the input shaft205 the input shaft 209 of the .balance unit 200 is in the correctposition to balance rthe beam 210.

Considering now the electrical circuit arrangement of the transmitterunit 41 shown in Fig. 3 of the drawings, the oscillator comprises theleft hand section of a dual triode vacuum tube 275, preferably of thecommercial type 12AU7. An unregulated alternating current supply whichis connected to the line input terminals 276 and 277 energizes the powertransformer 278 and the right hand section of the tube 275 functions asa rectifier so as to develop a unidirectional supply voltage of thepolarity indicated across the condenser y'279. This supply Voltage iscoupled through the oscillator tuning inductance 280 to the plate of theleft hand section of the .tube 275 and the cathode of this section isconnected through the resistor 281 to the negative side of the condenser279. A pair of resistors 282 and 283 are connected in series across thecondenser 279 so that a bridge circuit is formed with the oscillatortube forming one arm of the bridge, the other arms of this bridgecomprising the resistors 281, 282 and 283. The planar inductor 212 isconnected across the tuning condenser 285 and the upper end thereof iscoupled through the condenser 286 to the control grid of the oscillatortube, a grid leak resistor 287 being connected between the grid andcathode of this tube. The resulting circuit is well known in the art asthe tunedgrid, tuned-plate oscillator in which the small capacitancebetween the grid and plate of the triode vacuum tube provides therequired regenerative coupling 'between plate and grid circuits.

The beam 210 is deflected .in accordance with the movement of the inputshaft 209 and the position of the end 21.1 of the beam 210 with respectto the coil 212 is varied so as to change the tuning of the oscillator.Variation of the tuning of the oscillator changes the average biasestablished by the condenser 286 so that the average plate circuitimpedance of the oscillator tube is changed and the balance condition ofthe above described bridge circuit is altered. The transmitter outputcurrent signal is derived from the above described bridge circuit byconnecting one of the output terminals 290 to the junction of theresistors 282 and 283 and connecting the cathode of the oscillator tubethrough a resistor 291 and the upper portion of a potentiometer 292 tothe arm 293 thereof which is connected to ground, the other outputterminal 294 being also connected to ground. Accordingly, thetransmitter output current flows through the resistor 291, the upperportion of the potentiometer 292, the output device connected across theterminals 294 and 290 to the junction of the resistors 282 and 283. Forsorne applications it may be desirable to connect the output terminals294 and 290 to an existing circuit, such as a leased telephone line, forexample, for transmission to the control area. In such applications theterminal 294 and the arm of the potentiometer 293 are not connected toground so that the transmitter output current is unalected by anypotentials due to the telephone system which may be present on theoutput conductors 47, 48.

In order to stabilize the current output signal against changes in loadresistance and to provide an arrangement which is insensitive to linevoltage variations and changes in tube characteristics, a feedbackcurrent is derived from the above described bridge circuit and isapplied to the coil 214 in the correct polarity to apply a torque to thebeam 210 in opposition to the input torque applied through the shaft209. When a large feedback factor is employed the electromechanicalbalance unit 200 is made considerably more stable and the transmitterunit may be operated from an unregulated supply with out producingchanges in the output signal due to iluctuations in line voltage. Moreparticularly, the upper end of the feedback coil 214 is connected to thecathode of the oscillator tube and the lower end of the coil 214 isconnected through a resistor 296, a potentiometer 297 and a resistor 298to the bottom end of the potentiometer 292. Preferably, the values ofthe resistor 291 and the potentiometer 292 are relatively small ascompared .to the values ofthe resistors 296 and 298 and thepotentiometer 297 so that the major portion of the output current owsthrough the resistor 291 and the upper portion of the potentiometer 292.With this arrangement, the current flowing through the feedback coil 214may be adjusted as desired to provide an electrical span adjustment forthe transmitter unit 41 which may be adjusted independently of themechanical span adjustment described above, the shaft 299 of thepotentiometer 297 being accessible when the cover 203 is removed so asto simplify calibration of the instrument. Also, the resistors 291, 296and 298 and the potentiometers 293 and 297 are all of a low 4temperaturecoefiicient type so that the effects of temperature on zthe calibrationspring 242 `are compensated as will be described in more detail inconnection with a similar compensation network in the unit 42.

In accordance with an important feature of the invention, thetransmitter unit 41 is provided with a calibrated adjustment to permitmanual setting of the instrument to the speciic gravity of the liquidlevel being measured. More particularly, the potentiometer 292 isemployed as a. specific gravity control and the pointer knob 3699thereof cooperates with the scale Sill which is calibrated in terms ofthe speciiic gravity of the liquid. ln this connection it will beunderstood that a manually adjustable specific gravity control isparticularly desirable in processes where the level of the interfacebetween liquids of two different specific gravities is to be measured.In Calibrating the transmitter unit 4l to `a particular specic gravity,the pointer knob 360 is set to the specic gravity of the liquid to bemeasured as indicated on the specific gravity scale 301. At zero liquidlevel, the transmitter output current is adjusted to 0.5 milliampere bymeans of the zero adjustment screw 264 on the shaft 209 of theelectromechanical balance unit 200. With the displacer at maximum liquidlevel the transmitter output current is adjusted to 5.0 milliamperes byturning the slotted shaft 299 of the potentiometer 297 so that the spanof the transmitter unit `l1 is calibrated to the span of the liquidlevel displacer unit 4t?.

From the foregoing description it will be evident that variation of thearm of the potentiometer 292 changes the Value of the resistanceincluded in series with the output terminals 294 and 290 so that theinstrument 41 may be calibrated to the standard output current range of0.5 to 5.0 milliamperes with the different ranges of input movementwhich are obtained with liquids of different specific gravities. ln thisconnection, it will be understood that the differential pressuretransmitter 113 in the system shown in Fig. 2 may be substantiallyidentical to the transmitter 41 except for the above described specicgravity control network since both these units function with amechanical input signal and provide the same standard range of currentoutput.

Recording and Set point mit 42 As described heretofore in connectionwith the general operation of the process control system of the presentinvention, the recording and set point unit 42 performs the dualfunction of recording the value of the process variable and setting thecontrol point for process control of the system. Referring now to Figs.l2 to i9, 2l and 22 of the drawings, the components of the recording andlli set point unit 42 are mounted on separate chassis which are securedto a vertical frame 350 which is slidably positioned within a casing'351 provided with the hinged front cover 352 which may be opened topermit positioning of the set point indicator 53.

Considered generally, the recorder portion of the unit 42 comprises anelectrochemical balance 355 which is mounted on the vertical frame 350,a rotary solenoid 356 which is mounted on a sub-chassis 357 secured tothe frame 350 and a vertical scale, card chart driving mechanism 355which is also mounted on the sub-chassis 357. The rotary solenoid 356 isenergized by the balance unit 355 and its associated oscillator circuitand is connected to the recording lever 360 through the link 3611 sothat the recording pen 362 is moved vertically over the chart 50 inaccordance with the lever of the current input signal transmitted totheunit 42 from the transmitter 41. The recording lever 369 is alsoconneoted through the linkage 363 and 364 to the input shaft 365 of thebalance unit 355 so as to apply a mechani cal feedback signal to theunit 355. The lever 36th is pivotally connected to the vertical arm 366(Fig. 16) which is secured to a bracket 367 mounted on the verticalframe 350 by means of the thin resilient element 36S so that rotarymotion of the link 361 which is produced by the rotary solenoid 356,results in substantially straight line motion of the recording pen 362.The zero position of the recording pen 362 may be adjusted by means ofthe coarse zero adjustment 3619 md the fine zero adjust ment 370 so thatthe recorder unit may be calibrated mechanically to the standard currentinput signal.

Considered generally, the set point portion of the unit 42 comprises anelectromechanical balance unit 371 which is mounted on the verticalframe 35), the electrical circuit components for the balance unit 371being mounted on a separate sub-chassis 372 which is also mounted on theframe 350. The set point lever 373 which carries the set point indicator`53 at the forward end thereof is pivotally connected at 374 to thevertical arm 375 which is supported from the bracket 367 by means of thethin resilient element 376. The rear end of the lever 373 is connectedthrough the linkage 377 and 378 to the input shaft 379 of the balanceunit 371 so as to position the same in accordance with the posh tion ofthe set point indicator 53. The zero position of the set point lever 373may be adjusted by means of the coarse adjustment 380 and the tineadjustment 381 in a manner to be described in more detail hereinafter.

In order to adjust the position of the set point indicator 53 withrespect to' the scale 52 so as to determine the control point of theprocess, there is provided a worm screw 385, which is vertically mountedbehind the scale 52, and the set point lever 373 is provided with anoffset end portion 386 which rides against the threads of the screw 355.The set point indicator 53 is secured to an extension arm 387 which isset resiliently connected to the set point lever 373 by means of theflexible element 338 and the extension arm 387 carries a bracket 389which is provided with a U-shaped notch in the end thereof adapted toengage the threads of the screw 335 in the normal position of the arm387. A stop arm 393 is rigidly connected to the end of the set pointlever 273 and extends outwardly adjacent the extension arm 357. Withthis arrangement the position of the set point lever may be setapproximately to the desired control point when the front cover of theinstrument is open by grasping the extending portions of the members 337and 390 and pressing them together so that the notched bracket 339 isdisengaged from the screw 355 and the lever 373 may then be movedvertically up or down to the desired control point. The bracket 359 willrecngage the threads of the screw 335 when the members 357 and 390 orreleased. The position of the set point lever 373 may be adjustedaccurately by means of the knurled knob 391 which is positioned on thebottom end of the worm screw 385 and is accessible when the front coverof the instrument is open. Preferably, one full revolution of the knob391 causes the set point indicator 53 to move approximately 1% o'f fullscale so that a fine adjustment of the set point may be made after theindicator 53 is adjusted approximately to the desired set point bymoving the lever directly in the manner described above.

Considering now, the details of the coarse and iine zero' adjustmentprovided in the recording and set point linkages, it Will be understoodthat the coarse and tine zero adjustment 369 and 370 of the recorderlinkage are substantially identical to the coarse and fine zeroadjustments 380 and 381 of the set point linkage so that only one set ofadjustments need be described in detail. In Figs. 19, 21 and 22 there isshown the details of the coarse and fine zero' set point linkageadjustments and, referring to these figures, it will be seen that theinput shaft 379 of the set point balance unit 371 carries the toothedbushing 395 on the end thereof which is press tted on the end of theshaft 3'79 and carries the bracket 396 thereon. The bracket 396 is heldin place by means of a spring washer 397 which is positioned beneath thecollar 39S, and an adjusting gear 399 is mounted on the bracket 396 withthe teeth thereof engaging the teeth o'f the bushing 395. Accordingly,as the gear 399 is rotated the angular position of the bracket 396 withrespect to the shaft 379 may be varied to provide a coarse zeroadjustment of the linkage. The other end of the bracket 396 is pivotallysecured to the lever 378 by means of the rivet 400 and a transverselyextending adjusting screw 401 is mounted on the end of the lever 378'and threads through a block 402 secured to the end of the bracket 396.With this arrangement the adjusting screw 401 may be rotated by means ofthe knurled head 493 thereof so that the lever 378 pivots about therivet 400 to provide a ne zero adjustment of the set point linkage.

Considering now in more detail the balance units 355 and 371 and theelectrical circuit components associated therewith, reference may be hadto' Fig. 20 of the drawings wherein the electrical circuit diagram ofthe unit 42 is shown. Referring to this figure, the transmitter inputcurrent, which is transmitted over the conductors 47 and 48 from theunit 41, is applied to the input terminals 420 and 423 of the unit 42,the intermediate terminals 421 and 422 being connected togetherexternally of the unit so that the input circuits of the recorder andset point portions o'f the unit 42 are connected 1n series.

With regard to the recorder portion of the unit 42 the electricalcircuit arrangement thereof is similar in many respects to thecorresponding unit shown and described in detail in the copendingapplication of Charles G. Roper, Serial No'. 304,125, which was filed onAugust 13, 1952, now U.S. Patent No. 2,702,381, which issued on February15, 1955, and is assigned to the same assignee as the present invention,and reference may be had to this copending application for a detaileddescription of the electrical operation thereof. However, for thepurposes o'f the present invention, it may be stated generally that theelectromechanical balance unit 355 comprises a resiliently mounted beam425 adjacent one end of which there is provided `a planar inductor 426Which `varies the tuning of a high frequency oscillator including thetubes 427 and 428. The beam 425 carries an input coil 429 and a feedbackcoil 434i which are positioned within the air gap of the permanentmagnet structure 431. With regard to the mechanical arrangement of thebalance unit 355 it will be understood that this unit may besubstantially identical to the electromechanical balance unit 200 in thetransmitter 41, with the exception that both the input coil 429 and thefeedback coil 430 are mounted on the displaceable beam in the balanceunit 355. A portion of the input current applied to the terminals 422and 423 flows through the input coil 429 so as to produce a deflectionof the beam 425 in proportion thereto. For a given current input thebeam is balanced by the equal and opposing forces produced by the inputcurrent ilowing through the coil y429 and the sum of the mechanicalfeedback force applied through the input shaft 365 of the balance unit355 to the beam 425 and the fo'rce produced by any current flowingthrough the coil 430.

When the input current changes', the beam is deflected and theinductance of the coil 426 is varied so as to change the tuning of theoscillator and the output current of the oscillator, which flows throughthe field winding 435 lof the rotary -solenoid 356, is correspondinglyvaried. Accordingly, the armature 436 of the solenoid 356 assumes a newposition in accordance with the oscillator output current so that therecording lever 360 is positioned in accordance with the value of theinput current from the transmitter 41. Since the recording lever 360 ismechanically connected by means of the above described linkage to theinput shaft 365 of the balance unit 355, the armature 436 rotates untilthe spring loading on the beam 425 equals the force applied thereto bythe new input current level and the beam is again brought to a balancedcondition.

The current flowing through the field Winding 435 of the rotary solenoid356 is partially filtered by means of the filter condenser `437.However, in order to reduce the effects of static friction on thelinkage connecting the armature 436 to the recording lever 360, aresistor 438 is connected in `series 'with the condenser 437 so as toprovide a slight ripple in the current owing through the field Winding435. The value of the resistor 438 is so chosen that a slight vibrationof the armature 436 is produced which overcomes the eiects of staticfriction but is not of suicient 'amplitude to `al'fect the operation ofthe recording pen 362.

The input shaft 365 of the balance unit 355 is connected to the beam 425thereof lby means of a calibration spring -in la manner similar to thatdescribed in detail above in connetcion with the balance unit 200 in thetransmitter 41. As the ambient temperature rises, the calibration springbecomes weaker and accordingly introduces an error in the positioning ofthe recording lever 360. In order to compensate `for changes in thestrength of the calibration spring due to temperature variations, acompensating network is provided which includes the resistor 440, apotentiometer 441 and the resistor 442 which are connected in seriesacross the input coil 429 and a copper shunt resistor 446. The inputcurrent is applied between the armof fthe potentiometer 441 and thebottom end of the 'resistor 440. The resistors 444i and 442 and thepotentiometer 441 are all of the llow temperature coefficient type sothat the values thereof do not change appreciably with temperature.However, the resistance of the coil 429 and the shunt resistor 446,which are comprised of copper Wire, etc., increases with temperature. Asthe temperature increases, the currents flowing through the coil 429 andthe resistor 446 decrease since their resistance Values increases whilethe other resistances of the compensating network remain the same, andby properly proportioning the resistances of the compensation network,the current decrease in the input coil 429 for a given temperatureincrease can be made to exactly compensate for the weakening of thecalibration spring within the balance unit 355. Thus, if the totalseries resistance of the resistors 440 and 442 and the potentiometer 441is proportioned with respect to the combined parallel resistance of thecoil 429 and resistor 446 in the same ratio as; the temperature`coefcient of copper is to the tempera-ture coefficient of Phosphorbronze calibration spring, an exact compensation for temperatureVariations may be made. Furthermore, this compensation is independent ofthe range adjustment which is provided by adjusting 17Y the arm of thepotentiometer 441 so that the span of the instrument may be adjustedwhile maintaining the above described temperature compensation. In thisconnection it will be understood that at higher temperatures thepermanent magnet Istructure 431 becomes slightly weaker so that it isnecessary to take this into consideration in choosing the abovedescribed -ratio for the compensating network.

The mechanical feedback `force applied to the input shaft 365 of theunit 355 functions to stabilize the operation of the recorder. However,due to friction within the feedback linkage, hysteresis effects in the-rotary solenoid 356 and inertia eifects of the system, hunting of thesystem may be produced which causes undesired vibrations of therecording pen 362. In order to eliminate hunting, a Second feedbackforce, which is proportional to the rate of change of the output currentflowing through the eld winding 435, is applied to the beam 425. Morespecifically, a feedback coil 445 is wound on the core of the solenoid356 and the voltage produced thereacross is coupled to the feedback coil430 mounted on the beam 425. Accordingly, when the current through theeld winding 435 changes, a current proportional to the rate lof changeof the field current is applied to the feedlback coil 430 so as to applya second feedback force to the beam y4125 which is proportional to therate of change of the output current and compensates for the abovedescribed friction and hysteresis effects so that hunting issubstantially eliminated. Another important consequence of this feedback`force is the elimination of fluctuations in the posi-tion of therecording lever 360 which would be caused by uctuations of fthe supplyvoltage. Additional velocity damping of the beam is provided by theresistor 446 which `is connected 'across the coil 429. However, as notedabove, the resistor 446 is preferably formed of copper wire so that itwill have no effect on current division in the -above describedtemperature compensation network.

Considering now the electrical operation of the set point portion of theunit 42, it will be recalled that this unit is arranged to provide aD.C. voltage proportional to the deviation between the mechanicalsetting of the set point lever 373 yand Ithe transmitter output currentwhich is calibrated to the common scale 52. The polarity of ythe `outputvoltage varies depending upon whether the set point indicator is aboveor 'below the input current as indicated by the recording indicator 51.The input shaft 379 of the :balance unit 371 is positioned in accordancewith the setting of the set point lever 373 so that a torque is yappliedto the beam 450 thereof in proportion to the position of the set pointlever. A portion of the transmitter output current flows th-rough theinput coil 451 of the balance unit 371 so that a force is eX- erted onthe beam 450 in opposition to the mechanical input due to the set pointlever. If the set point indicator 53 is adjusted to the value of theinput current, as indicated by the pointer 51, the forces on the beam450 are balanced.

One end of the beam 450 is positioned adjacent a planar inductance 452and when the beam is balanced the value of the inductance 452 is such asto tune the oscillator circuit, which includes the left hand section ofthe double triode tube 453 so that zero output Voltage is producedbetween the output terminals 454 and 455. The oscillator tube acts asone arm of a bridge circuit which includes the resistors 456, 457 and458 and when the beam 450 is balanced the plate impedance of theoscillator tube is of the correct value to balance this bridge circuitso that zero voltage appears between the output terminals 454 and 455. Afeedback current proportional to the output voltage is impressed uponthe feedback coil 459 through the series resistor 460. This feedbackestablishes the relationship between the output voltage and thedifference in the torques resulting from the current in the coil 451 andthe position of the set point lever 373 and causes the output voltage tobe substantially unaffected by changes in line voltage, and changes inthe parameters of all components except the balance unit 371 and theprecision resistor 46d, as well as changes in the load connected acrossthe output terminals 454 and 45S. However, it will be understood thatwhen the system is at the control point, i.e., when zero output voltageis produced between the terminals 454 and 455, no feedback current iiowsthrough the coil 459 since under these conditions the beam 45@ isbalanced. IIn this connection it is noted that the transmitter .W5 inthe system of Fig. 2 is electrically similar to the set point portion ofthe unit 42. However, in the transmitter the thermocouple 162 provides acurrent input signal to the coil 451 and no mechanical set point linkageis required. lIn the alternative an arrangement such as shown anddescribed in detail in the United States patent to C. G. Roper, No.2,614,163, issued on `@ctober 14, 1952, may be employed for thetransmitter 3105.

-If the position of the set point lever 3'73 is varied, or the value ofthe input current changes, the beam is deflected so as to change thevalue of the inductance 452, thereby changing the tuning of theoscillator tube and the plate impedance thereof so that the bridgecircuit becomes unbalanced and an error voltage is produced between theterminals 454 and 455 which is proportional to the deviation of theinput current from the mechanical set point and has a polarity whichdepends on the direction of the deviation. The input current impressedupon the input terminals 42) and 421 is supplied to the input coil 451through a temperature compensating network which includes the resistor465, the potentiometer 466 and the resistor 467 which are connected inseries across the input coil 415i, the input current being appliedthrough the arm of the potentiometer 466. NVith this arrangement,compensation is obtained for variations in the strength of thecalibration spring which connects the shaft 379 to the beam 450 due totemperature in the manner similar to that described above in connectionwith the compensation network in the recorder portion of the unit 42. Ifdesired, a copper wire damping resistor 461 may be connected across theinput coil 451 to reduce the amount of motion resulting from rapidlychanging applied torques by means of velocity damping.

In order to permit grounding of one of the output conductors 66, 67which transmit a direct current signal from the control area to theprocess area, the conductors S5 and 56 which connect the unit 42 and thecontroller 43 are not connected to ground for reasons to be described inmore detail in connection with the controller 43. Accordingly, the DC.potential level on the conductors 55 and 56 may vary considerablydepending upon the output current level from the controller 43. in theset point portion of the unit 42, the conductor 5S is connected to thecathode of the oscillator tube and the beam 450 is connected to groundthrough its supporting elements. It has been found that if the gridinductor 452 is connected directly to the cathode of the oscillatortube, the voltage between the beam 450 and the adjacent planarinductance 452 develops an electrostatic force of suicient magnitude todeflect the beam 45t) and thereby produce an erroneous output voltagebetween the terminals 454 and 455. In order to eliminate this diiicultyboth sides of the grid inducto-r 452 are capacitively coupled to theoscillator input circuit and this inductor is connected to ground. Thus,one end of the inductor 452 is coupled through the condenser 470 to thegrid of the oscillator tube and the other end of the inductor 452 iscoupled through the condenser 471 to the cathode of the oscillator tube,a resistor 472 of relatively high value being connected from thejunction point of the inductor 452 and the condenser 471 to ground. Withthis arrangement both sides of the grid inductor 452 are RF coupled tothe oscillator input circuit and this inductor is connected to groundthrough the resistor 472 so that changes in the D.C. potential of

